1. Field of the Invention
The present invention relates to an optical sheet which is suitably used in displays such as LCD (liquid crystal display), EL (electro-luminescence) display, FED (field emission display), etc. and particularly having excellent effects on prevention of luminance irregularity, improvement in contrast of the displays, and having a wide visual field angle.
2. Description of the Related Art
Displays such as LCD, EL, FED, etc. have been remarkably developed in recent years. In particular, the LCD is spread in various fields, such as for notebook personal computers, portable terminals, etc., and the expectation to the future is big for them.
This LCD is roughly classified into a reflection type one and a transmission type one by how to take light for illuminating a liquid crystal panel thereinto. In the reflection type is a system that a reflection plate, on which an aluminum or silver film having a high reflectivity has been stuck, is arranged on the back surface of the liquid crystal panel, and the light which is struck on it from the surface side of the display is reflected by the reflection plate to illuminate the liquid crystal panel, thereby liquid crystal images being formed. On the other hand, the transmission type is a system that the liquid crystal panel is illuminated by a back light unit arranged on the back surface of the liquid crystal panel.
In the reflection type LCD, in order to prevent contrast from being deteriorated by the ground color of the aluminum or silver, it is conducted to interpose a medium that moderately diffuses the light between the liquid crystal panel and the reflection plate to bring the background color near to a paper white color.
In the transmission type LCD, in order to prevent visibility from being deteriorated by a light-scattering printing pattern formed on an acrylic light-guide plate making up the back light unit, it is so constructed that a medium that moderately diffuses light is interposed between the liquid crystal panel and the back light unit to illuminate the liquid crystal panel by uniform surface light.
As mentioned above, the light-diffusing medium (referred to as “light diffuser”, hereinafter) is generally used in both systems of the reflection type and the transmission type. As such light diffusers, are known an interior light diffuser that transparent fine particles having a different refractive index from that of the transparent resin are dispersed in the transparent resin layer, so that the light diffuses in the inside of the layer, an exterior light diffuser that the surface of a resin layer is roughened to form ruggedness on the surface, thereby the light diffusing on the surface, and an interior-exterior light diffuser that a part of transparent fine particles projects on the surface of the transparent resin layer so that the light diffuses both in the interior and on the exterior.
However, the above mentioned interior diffuser has a problem that the light transmittance is generally low. In the interior light diffuser, since the fine particles are dispersed at random in the transparent resin, the light ray passes through a large number of fine particles to cause extra light scattering when the light passes through the transparent resin. As a result, the light transparency becomes low.
The exterior light diffuser and the interior-exterior light diffuser have each a problem that the irregularity in the light diffusivity is often observed, since the degree of light diffusivity depends on the irregularity of the surface. Though it is necessary to control the height and pitch of the ruggedness in the surface so as to have a fixed state in order to reduce the diffusive irregularity, it is very difficult to do so. In addition, there is a problem that the adherent fouling is hard to be removed because of surface irregularity.
The present invention has been made in the light of the above described circumstances. An object of the present invention is therefore to provide an optical sheet having and sufficiently exhibiting high light transparency and uniform light diffusivity.